Intelligence as manifested in life is predominantly about the use of cognitive tools.
This can include empathy and the ability to speculate and reconstruct yourself to identify with the feelings of another, this can include the ways you map out information, the dimensions in which you are capable and willing to turn th around to view the information map from different perspectives, the scope and abstraction level of the patterns your mind is able to form & recognize. This can a more meta layer of self awareness, such as the patience and will power to self examine why you think & feel what you do and where it puts you in the mental maps of others while detaching yourself from the distractions of righteousness, and it can include logic and rhetoric, and math and linguistic capacities and so much more. Perhaps most valuable (If i am somewhat biased), it includes this: a meta level of understanding of this very mind space, the tool belt which your mind yields, their construction, formulation and value.
From this perspective, to say that intelligence about capacity would be to assume we even have cognitive tools at our disposal that can reach the upper limit of anyone's capacity, maximize the human potential.
and I believe we have very strong evidence against this from the research of James Flynn, who has tracked the increase in IQ results & multiple intelligence tests down to an increase in 3 basic mental tools: Seeking classification, using logic on abstractions & taking the hypothetical seriously. Those are cognitive 3 tools that continue to improve over time at a steady rate without any visible cognitive capacity suffering as a result, which to me suggest that this trend does not obey the redistribution effects of a limited resource pool (Moving points from capacity A to capacity B), and that we are not getting any decrease in return or hitting any walls, meaning there is no visible scarcity effect in the ability to grow. Most importantly, common sense dictates that we should not be as ridiculously arrogant as to assume that our society has fulfilled the human potential better then all possible human societies.
Much like cultural analysis at the face of rapid technological change, IQ tests are never going to be able to fully catch up to all the mental tools we make, at least not until we get close to the capacity. But they can do exactly what they already do - make a focus test on our ability to use recognizable tools.